1. Field of the Invention
The present invention relates to an extreme ultraviolet (EUV) light source apparatus to used as a light source in exposure equipment.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress toward finer fabrication. In the next generation, microfabrication at 70 nm to 45 nm, further, microfabrication at 32 nm and beyond will be required. Accordingly, in order to fulfill the requirement for microfabrication at 32 nm and beyond, for example, exposure equipment is expected to be developed by combining an EUV light source for radiating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made higher, that the light of only the particular waveband can be radiated by selecting the target material, and that an extremely large collection solid angle of 2π to 4π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure such as electrodes surrounding the light source. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography, which requires power of more than several tens watts.
In the LPP type EUV light source apparatus, EUV light is radiated on the following principle. That is, by supplying a target material into a vacuum chamber by using a nozzle and applying a laser beam to the target material, the target material is excited and turned into plasma. Various wavelength components including extreme ultraviolet (EUV) light are radiated from the plasma generated in this manner. Then, the EUV light is reflected and collected by using a collector mirror for selectively reflecting a desired wavelength component (e.g., 13.5 nm) among them, and inputted to an exposure unit. For example, as a collector mirror for collecting EUV light having a wavelength near 13.5 nm, a mirror having a reflecting surface on which molybdenum (Mo) and silicon (Si) thin films are alternately deposited is used.
In the LPP type EUV light source apparatus, there is a problem of an influence of ion particles and neutral particles emitted from the plasma. These particles (debris) fly to the surfaces of various optical elements such as an EUV collector mirror within the chamber. The fast ion debris with high energy erode the surfaces of the optical elements. On the other hand, slow ion debris and neutral particles are deposited on the surfaces of the optical elements. Due to the influence of the debris, the reflectivity of the surface of the optical elements becomes lower to be unusable.
As a related technology, Japanese Patent Application Publication JP-P2005-197456A discloses an extreme ultraviolet light source apparatus having magnetic field generating means for generating a magnetic field within collective optics to trap ion debris so that the ion debris emitted from plasma may not collide with a collector mirror.
However, in JP-P2005-197456A, because of moving of the ion debris with high energy along with the magnetic flux, the ion debris may collide with a target nozzle provided on the magnetic field axis. If the ion debris collide with the target nozzle, the nozzle is sputtered and changed in the shape of tip of the nozzle, and thereby, positional stability of droplets may be deteriorated and materials sputtered from the nozzle may adhere to the collector mirror and so on and reduce the reflectivity. Further, debris such as neutral particles cannot be trapped by the magnetic field but adhere to the surfaces of the collector mirror, and then, it causes the reflectivity reduction.